1. Field of the Invention
The present invention relates to an image forming apparatus provided with a developing apparatus containing a developer for developing an electrostatic latent image, that image forming apparatus is used for forming a latent image on an electrostatic latent image bearing member such as an electrophotographic photosensitive member or a static recording dielectric member and converting the latent image to a visible one. It also relates to an apparatus unit and a facsimile apparatus making use of such an apparatus.
2. Related Background Art
In recent years, uses of electrophotographic printers are being rapidly spread as computer output apparatus, and also spread as printers useful in office automation and as facsimile image reproducing apparatus. These printers are required to have a high print quality.
A laser-beam printer (LBP) that is prevailing in the printers using an electrophotographic system is an output apparatus by which the on-off of a semiconductor laser, corresponding with the output information from a computer, is written as a digital latent image onto a photosensitive drum through a rotating multifacer mirror, and an image is printed on a recording sheet by an electrostatographic process.
In an electrophotographic apparatus in which a latent image is comprised of fundamental picture elements (hereinafter "dots" as in the case of an LBP, the light output to a photosensitive member pertains to the formation of a digital latent image composed of an on-off binary, and hence the edge effect becomes predominant when the latent image is developed.
The edge effect is a phenomenon in which electric lines of force concentrate at the boundary between an exposed area and an unexposed area of a latent image to cause an apparent increase in surface potential of a photosensitive member, resulting in an increase in image density at the boundary. It has been hitherto considered that this phenomenon is undesirable and should be avoided, since it brings about a non-uniformity in a solid image (i.e, an increase in image density at edges).
On the other hand, in image forming methods in which a latent image is composed of picture elements of 50 .mu.m to 150 .mu.m in size, the part affected by the edge effect is larger than that of usual analog images, and hence the edge effects are positively utilized to obtain a developed image with a good line reproduction and a high image density.
As a special tendency in the development of edge portions, a gradient of charge quantity is produced corresponding with a gradient of potential, unless the charge quantity of a developer is sufficiently high. Therefore, toner particles with a large charge quantity are selectively used in preference, and toner particles with a small charge quantity tend to remain unused in a developing assembly, resulting in a deterioration due to running.
This tendency in digital latent image system becomes more remarkable, in image forming systems used for the purpose of printers, such as laser beam printers and liquid crystal shutter printers, because character images are mainly output. Developers conventionally used in such digital latent image systems, because of the special tendency in edge development, often cause a deterioration problem of image quality after running of a large number of sheets, as well as a thinner line-image problem in an environment of high humidity.
In reversal development carried out in image forming apparatus such as laser beam printers, smaller electrical charge is present at image areas and a greater electrical charge at the background on a photosensitive member. Hence, when conventional toners are used and toner particles having a smaller charge quantity are present, the toner particles are attracted to the background having a greater electrical charge. Thus, prevention of this reversal fog has been one of most important subjects in the past in this electrophotographic process.
In the meantime, for controlling the charge quantity (triboelectricity) of a dry one-component magnetic toner, it is known to externally add to a toner a material as exemplified by silica treated in gaseous conditions (hereinafter "dry silica") and silica treated under wet conditions (hereinafter "wet silica").
For example, the charge quantity required for a developer can be increased by dry-mixing a dry negative silica that exhibits strong negative characteristics (a silica obtained by adding 10 parts by weight of hexamethyldisilazane (HMDS) to 100 parts by weight of dry silica having a BET specific surface area of 100 m.sup.2 /g), in a negatively chargeable magnetic toner containing 100 parts by weight of a styreneacrylate type copolymer and 60 parts by weight of magnetite. When, a layer such developer is formed on a sleeve comprised of a cylindrical member made of a metal such as aluminum or stainless steel, it becomes possible to increase image density and also to obtain smoother images compared with a developer containing no silica.
In an environment of high humidity, however, it is not easy to obtain a satisfactory image density by only a conventional silica externally. In an environment of high temperature and/or high humidity (in particular, in an environment of high humidity), the silica contained in a developer causes a phenomenon of moisture absorption to bring about a decrease in charge quantity of the developer. It has been common that images with a good image density can be obtained in a low-humidity environment or a normal environment, but those with a low image density and coarseness are obtained in an environment of high humidity.
To cope with this problem, it has been attempted in several instances to subject the silica to a hydrophobic treatment so that the absorption of moisture in an environment of high humidity can be prevented.
However, with a developer, for example, comprising a negatively chargeable toner and the externally added negative silica which has been subjected to hydrophobic treatment, print patterns may remain on a developing sleeve sometimes to damage the reproduction of good line images. This phenomenon often occurs in an environment of low temperature and low-humidity, in particular, low humidity. According to the experiments and studies by the present inventors the mechanism of this phenomenon is greatly concerned with a layer of a fine powder (particle diameter: 5 to 6 microns or less), formed on the sleeve. A marked difference in particle size distribution at a lowermost layer of the toner on the developing sleeve is present between toner-consumed areas and toner-unconsumed areas. At the lowermost layer of the toner in the unconsumed areas, a fine powder layer is formed. Since the fine powder has a large surface area per volume, it prossesses a larger triboelectric charge quantity per mass than particles having a large particle diameter, and thus electrostatically strongly bound to the sleeve because of the mirror force of the fine powder itself. Hence, the toner present on the part at which the fine powder layer has been formed can not be sufficiently triboelectrically charged, tending to result in a lowering of development capacity in reproducing line images faithful to latent images.
The developer containing the silica pretreated for hydrophbicity shows a stable charge in an environment of high humidity, but causes an excessive increase in charge quantity in an environment of low humidity. In particular, the fine powder may become charged up to bring about a partial lowering of developing power. This tends to makes it difficult to faithfully develop the digital latent image, such as line images which are composed of dots.
A developing method in which a latent image formed on the surface of a photosensitive drum (an electrostatic latent image bearing member) is converted to a visible image using a magnetic toner as one-component developer, hitherto includes a method comprising i) imparting to the magnetic toner particles a charge with the opposite polarity to that of an electrostatic latent image formed on a photosensitive drum from development standard potential by mutual friction of the magnetic toner particles and also friction between a sleeve serving as a developer-carrying member and the magnetic toner particles, ii) transporting the magnetic toner particles thinly spread on the sleeve to a developing zone defined by the photosensitive drum and the sleeve, and iii) in the developing zone, transporting the magnetic toner particles by the action of a magnetic field of a magnet set inside the sleeve, thereby converting the electrostatic latent image on the photosensitive drum to a visible image.
The above conventional technique, however, has been involved in the problem that an area with a low density in a stripe appears when the same pattern is repeatedly printed. FIG. 2 diagrammatically illustrates an image having such an area.
This is a phenomenon in which lines of characters become thin in the case of character images and density becomes low in the case of halftone or solid black images. This phenomenon is hereinafter called "fading".
The above phenomenon of fading is particularly remarkable in an environment of high temperature and high humidity where the charge of a developer tends to lower.